1. Field of the Invention
This invention relates generally to apparatus for measuring the flow of gas or air through a conduit, and more specifically to such apparatus for use in the gas or air mains of a blast furnace.
2. Prior Art
It has been known in the field of fluid flow measurement that the flow of gas or air through a conduit may be measured by creating an orifice in the conduit through which all of the fluid in the conduit must flow, and locating a pair of pressure differential tubes, or impulse tubes, on opposite sides of the orifice. Apparatus is connected to the tubes for measuring the pressure differential in the tubes created by the flow of fluid through the orifice. This pressure differential may then be converted into a measurement of the rate of flow of the fluid through the conduit. One apparatus of this type in which the ends of the tubes are located in the side of the conduit is disclosed in U.S. Pat. No. 2,927,462. In the type of apparatus disclosed in that patent, the orifice is created in the center of a plate which is mounted in the conduit. Another type of pressure differential producing apparatus is disclosed in U.S. Pat. No. 2,687,645. In that patent an annular opening is formed in a plate which is mounted in the conduit, and the pressure differential tubes are mounted such that the open ends of the tubes are located in close proximity to opposite sides of the center of the orifice plate. In apparatus of the type shown in U.S. Pat. No. 2,687,645, it has been found that the spokes which connect the circular center portion of the plate to the annular outer portion of the plate interfere with the air flow and thus limit the accuracy of the flow readings obtainable. Also, the orifice plate in an apparatus of this type is mounted between flanges in the conduit, and the tubes are mounted directly in the walls of the conduit. In this arrangement, it is not feasible to remove the orifice plate and tubes for inspection of the apparatus and it is, therefore, necessary to provide an inspection window in the conduit for this purpose. In addition, because the orifice plate and the tubes are mounted separately in the conduit, it is difficult to ensure the close tolerances necessary in the placement of the open ends of the tube proximate the center of the orifice plate.
The present invention eliminates the multiple spoke supports holding the circular center plate, and thus reduces or eliminates the variable turbulances caused by the supports. The elimination of such supports also allows for use of a single plate with a maximum circular sharp-edge exposure to the fluid flow for increased accuracy of measurement. The integral mounting of the tubes and plates provides for accurate, positive and permanent impulse tube settings in relation to the orifice plate and allows the apparatus to be installed in a short pipe section or elbow. The lowered turbulance factor permits a higher downstream pressure recovery rate and eliminates the need for extra straight lengths of main pipe required by the conventional, adjustable type impulse tubes. Thus the present apparatus allows for uniform pressure differential readings through a full range of flow rates. The assembly can also be easily removed from the conduit, eliminating the need for inspection openings in the conduit.